Robust regulation of infinite-dimensional systems

One of the classical problems in control theory is the robust output regulation problem. The output of a system is required to asymptotically follow a given reference signal, despite disturbance signals and perturbations in the system's parameters. Wonham & Francis, and Davison on the other hand, developed independently the theory and construction of finite-dimensional robust controllers. Wonham's approach was based on geometric theory while Davison introduced a combination of servo-compensator and stabilizing compensator. They discovered the famous Internal Model Principle, which states that a robust feedback controller must contain a copy of the dynamics of the exosystem generating the reference and disturbance signals.